The present invention relates to radiation therapy, and more particularly, to a system and method for efficiently delivering radiation treatment.
Radiation emitting devices are generally known and used, for instance, as radiation therapy devices for the treatment of patients. A radiation therapy device generally includes a gantry which can be swiveled around a horizontal axis of rotation in the course of a therapeutic treatment. A linear accelerator is located in the gantry for generating a high energy radiation beam for therapy. This high energy radiation beam can be an electron beam or photon (X-ray) beam. During treatment, this radiation beam is trained on one zone of a patient lying in the isocenter of the gantry rotation.
To control the radiation emitted toward an object, a beam shielding device, such as a plate arrangement or a collimator, is typically provided in the trajectory of the radiation beam between the radiation source and the object. An example of a plate arrangement is a set of four plates that can be used to define an opening for the radiation beam. A collimator is a beam shielding device which could include multiple leaves, for example, a plurality of relatively thin plates or rods, typically arranged as opposing leaf pairs. The plates themselves are formed of a relatively dense and radiation impervious material and are generally independently positionable to delimit the radiation beam.
The beam shielding device defines a field on the object to which a prescribed amount of radiation is to be delivered. The usual treatment field shape results in a three-dimensional treatment volume which includes segments of normal tissue, thereby limiting the dose that can be given to the tumor. The dose delivered to the tumor can be increased if the amount of normal tissue being irradiated is decreased and the dose delivered to the normal tissue is decreased. Avoidance of delivery of radiation to the organs surrounding and overlying the tumor determines the dosage that can be delivered to the tumor.
Typical radiation therapy machines deliver treatment in the form of xe2x80x9cintensity modulated radiation therapy.xe2x80x9d Essentially, multiple coplanar beams whose fluence profiles are modulated in two dimensions are used to achieve a uniform high dose region that closely conforms to a target volume in three dimensions and thus spares normal tissue regions.
For example, FIG. 1A and FIG. 1B illustrate a discrete intensity map 100 having a footprint 102 that is to be delivered in treatment.
The direction X denotes a dose level to be applied. FIG. 1A illustrates the intensity map; FIG. 1B illustrates the map applied on the patient 104.
In general, IMRT may be delivered in any of three ways: static IMRT (also known as xe2x80x9cStep and Shootxe2x80x9d); Dynamic IMRT (also known as xe2x80x9csliding windowxe2x80x9d); and IMAT (arc IMRT).
FIG. 2A and FIG. 2B illustrate static or sequential IMRT. In particular, shown are a multi-leaf collimator 200 defining a shape 204 and an associated fluence profile 203. As shown, the leaves 202a, 202b of the MLC 200 define an opening 204 that is to be delivered. Radiation is on for a predetermined period while the leaf settings are as shown. The particular leaf setting 204 corresponds to a step of the fluence profile. Thus, the fluence profile consists of a plurality of such settings built up in a stepwise fashion.
FIG. 3 illustrates sliding window IMRT. Shown at 302 is a track or xe2x80x9cside viewxe2x80x9d of the intensity map, for a given set of two opposing collimator leaves. In dynamic IMRT, radiation is ON while the leaves are moving. Thus, shown in 303 is a diagram of a particular leaf motion corresponding to the map 302 over time. The leaf assumes various positions 304a . . . 304n over time, and defines various openings 306a . . . 306n correspondingly. Thus, each level 308a, 308b, 308c and so on is built over time, with the peaks 310, 312 being built separately.
In this technique, a variable width slot moves across the field and exposes every point on the intensity map to create slopes. At both the beginning and ending of the treatment, the collimator is closed. The leaves are closed simultaneously as the radiation is turned off. This can result in delivery of excess radiation, if the closing of the leaf and the turning off of the radiation are not synchronized.
Finally, in arc IMRT, the radiation stays on while the leaves are moving and the gantry is rotating at constant speed. While using this technique, one intensity level is delivered per gantry revolution.
These and other problems in the prior art are overcome in large part by a system and method for control of radiation therapy delivery according to the present invention.
A dynamic IMRT scheme according to an embodiment of the invention defines a RAD ON/RAD OFF cycle as an IMRT segment. Every set of opposing leaves in the collimator produces an IMRT profile or track. According to such an embodiment, at least one of the opposing leaves moves toward the other to produce the given track. When a track is complete, the opposing leaves remain together until the end of the segment. The dose rate remains constant during the segment.